An ionic liquid is a salt in a liquid state. Ionic liquids are also commonly known as liquid electrolytes, ionic melts, ionic fluids, fused salts, liquid salts, or ionic glasses. Ionic liquids are used as solvents or electrolytes due to their desirable properties, such as absence of a vapor pressure, high heat resistance and broad liquid temperature range, non-flammability, chemical stability, high ionic conductivity, high decomposition, and handleability in air.
As described in “Phosphazene-Based Ionic Liquids: Synthesis, Temperature-Dependent Viscosity, and Effect as Additives in Water Lubrication of Silicon Nitride Ceramics,” Omotowa et al., Inorg. Chem. 43:5466-5471 (2004), phosphazene-based ionic liquids have been investigated for use as lubricants for aircraft gas turbine engines and as additives in water lubrication of silicon nitride ceramics.
Phosphazene-based ionic liquids have also been proposed for use in electrolyte solutions. U.S. Pat. Nos. 7,718,826 and 7,951,495 describe phosphazene-based ionic liquids where one or more quaternized nitrogen atoms are directly bonded to phosphorus atoms of the phosphazene compound. Thus, the ionic liquids include a charge localized at the quaternized nitrogen atom. The specific examples of phosphazene-based ionic liquids include direct phosphorus-fluorine bonds, which are labile and unstable under cell voltage conditions. The free fluoride released is an aggressive reactant and detrimental to cell performance.
Electrolyte solutions used in lithium-ion batteries are typically unstable at high temperatures and high voltages. Over time, the electrolyte solution turns into a tar-like material at high temperatures, which has precluded rapid deployment of lithium-ion batteries in vehicular applications, such as in hybrid electric vehicles (HEVs) and plug-in type hybrid electric vehicles (PHEVs). Improved electrolyte solutions are needed to advance the usefulness of lithium-ion batteries.